In the manufacture of semiconductor devices, ion implantation is used to dope semiconductor wafers with impurities. Ion implanters or ion implantation systems treat semiconductor wafers with an ion beam, to produce n or p-type doped regions or to form passivation layers in the wafers during fabrication of integrated circuits. When used for doping semiconductors, the ion implantation system injects a selected ion species to produce the desired extrinsic material. Implanting ions generated from source materials such as antimony, arsenic or phosphorus results in n type extrinsic material wafers, whereas if p type extrinsic material wafers are desired, ions generated with source materials such as boron, gallium or indium may be implanted. Ion implanters are also used for other applications in the manufacture of semiconductor devices that require implanting additional species. For example, germanium is implanted into silicon wafers to preamorphize the silicon and prevent ion channeling in subsequent implantation steps.
Ion implantation systems typically include an ion source for generating positively charged ions from such ionizable source materials. The generated ions are extracted from the source and formed into an ion beam, which is directed along a predetermined beam path in a beamline assembly to an implantation station, sometimes referred to as an end station. The ion implantation system may include beam forming and shaping structures extending between the ion source and the end station, which maintain the ion beam and bound an elongated interior cavity or passageway through which the beam is transported en route to one or more wafers or workpieces supported in the end station. The ion beam transport passageway is typically evacuated to reduce the probability of ions being deflected from the predetermined beam path through collisions with air molecules.
The charge-to-mass ratio of an ion affects the degree to which it is accelerated both axially and transversely by electric or magnetic fields. Ion implantation systems typically include a mass analyzer in the beamline assembly downstream of the ion source, having a mass analysis magnet creating a dipole magnetic field across the beam path in the passageway. This dipole field operates to deflect various ions in the ion beam via magnetic deflection in an arcuate section of the passageway, which effectively separates ions of different charge-to-mass ratios. The process of selectively separating ions of desired and undesired charge-to-mass ratios is referred to as mass analysis. Using mass analysis techniques, the beam imparted on the wafer can be made very pure since ions of undesirable molecular or atomic weight will be deflected to positions away from the beam path and implantation of other than desired materials can be avoided.
High energy ion implantation is commonly employed for deeper implants in a semiconductor wafer. Conversely, high current, low energy ion beams are typically employed for high dose, shallow depth ion implantation, in which case the lower energy of the ions commonly causes difficulties in maintaining convergence of the ion beam. In particular, high current, low energy ion beams typically include a high concentration of similarly charged (positive) ions which tend to diverge due to mutual repulsion, a space charge effect sometimes referred to as beam blowup. Beam blowup is particularly troublesome in high current, low energy applications because the high concentration of ions in the beam (high current) exaggerates the force of the mutual repulsion of the ions, while the low propagation velocity (low energy) of the ions expose them to these mutually repulsive forces for longer times than in high energy applications. Space Charge Neutralization is a technique for reducing the space charge effect in an ion implanter through provision and/or creation of a beam plasma, comprising positively and negatively charged particles as well as neutral particles, wherein the charge density of the positively and negatively charged particles within the space occupied by the beam are approximately equal. For example, a beam plasma may be created when the positively charged ion beam interacts with residual background gas atoms, thereby producing ion electron pairs through ionizing collisions during beam transport. As a result, the ion beam becomes partially neutralized through interaction with the background residual gas in the beam path.
The ion beam typically propagates through a weak plasma that is a byproduct of the beam interactions with the residual or background gas. A plasma has the property that it shorts out electric fields in very short distances due to the high mobility of the electrons. The plasma therefore tends to neutralize the space charge caused by the ion beam by providing negatively charged electrons along the beam path in the passageway, thereby largely eliminating transverse electric fields that would otherwise disperse or blow up the beam. However, at low ion beam energies, the probability of ionizing collisions with the background gas is very low. Consequently, little or no space charge neutralization of low-energy beams occurs in implanters as a result of plasma produced by beam interactions with residual gas. In the dipole magnetic field of a mass analyzer, moreover, plasma diffusion across magnetic field lines is greatly reduced while the diffusion along the direction of the field is unrestricted. As a result, introduction or maintenance of additional plasma for space charge neutralization is difficult in the mass analyzer portion of an implanter, since such plasma is quickly diverted along the dipole magnetic field lines to the passageway sidewalls. In addition, low energy implantation systems typically suffer from electrons being lost to the sidewalls along the beamline assembly, which reduces the number of such electrons available for space charge neutralization.
Furthermore, in scanned-beam systems where an ion beam is scanned using alternating electric or magnetic fields, plasma generation is difficult because of the time-varying nature of plasma diffusion through such fields. Such systems often include scanning apparatus for scanning the ion beam in one direction while the target wafer or wafers are scanned in an orthogonal direction to provide for uniform implantation of the target wafers. The beam is typically scanned at some point along the beamline using varying electric or magnetic fields, and focusing apparatus (e.g., sometimes referred to as a parallelizer) is provided downstream of the scanning apparatus to make the scanned beam parallel. For low energy high current beams, magnetic fields are preferred for scanning and parallelizing the beam. However, the time-varying scanning makes beam confinement through beam-generated plasma difficult, wherein the beam moves away from the spatial location where plasma is being generated. Thus, the density of beam-generated plasma within the volume the beam is moving into is much lower than the plasma density in the volume the beam is moving out of. Thus, there is a need for scanned and non-scanned beam ion implantation systems and methods for enhancing space charge neutralization and preventing or reducing beam blowup.